This invention relates to a method of and an arrangement for combusting black liquor from a pulp mill in a recovery boiler, whereby the heat economy of the process has been specifically enhanced.
Conventionally, the black liquor from a pulp mill has been combusted at a dry solids content of approximately 63-75%. It is advantageous to remove as much water as possible from the black liquor before feeding the liquor into a recovery boiler, because the vaporization of the water in the furnace cools the furnace, slows down the combustion process and, due to an increased heat capacity, impedes the recovery of the heat that has become bound in the flue gases during the combustion. At present, a purpose is to concentrate the black liquor to a considerably higher dry solids content (i.e. 80-90%), which provides several benefits in the combustion of black liquor. Liquor droplets having a high dry solids content dry faster and with a smaller amount of heat and they also burn quicker. A decreased flue gas flow allows the installation of smaller and inexpensive heat transfer surfaces and the possibility to position the post-heat transfer surfaces of the boiler so that more electricity can be produced. Also the risk of blocking in the heat recovery section of the recovery boiler will be smaller. An increase in the dry solids content raises the temperature at the bottom part of the furnace, whereby emissions of sulfur dioxide are lesser and the level of reduction in the char bed increases.
For the recovery the black liquor is concentrated in a multi-effect evaporation plant. Usually the evaporation plant operates so that hot vapor, e.g. live steam, is fed as heating medium into an evaporator at the highest pressure, said vapor boiling down the liquor on the liquor side of the evaporator, thus generating vapor at a lower pressure than the vapor directed to the vapor side of the evaporator. The vapor thus generated is used to heat an evaporator at the next highest pressure while the liquor flows through the evaporation plant countercurrent in relation to the heating steam. Correspondingly, the same process takes place in the third effect and in the fourth effect etc. until the pressure and the temperature of the liquor discharged from the last effect in relation to the flow of the vapor are so low that it is technically and economically unprofitable to continue. The vapor having the lowest pressure is usually condensed in a condenser by using water or air. Typically an evaporation plant has 5 to 7 effects.
Treatment of black liquor at a high dry solids content involves certain problems. The viscosity of the black liquor increases exponentially as the dry solids content increases. In order to overcome the resistance caused by the viscosity, the black liquor has to be evaporated and transferred to a recovery boiler at a high temperature and at a super-atmospheric pressure. When the dry solids content of the black liquor is above 80%, the viscosity of the liquor is so high and the boiling point rise so considerable that ordinary back pressure steam is not sufficient for the final concentration. Medium pressure steam (typically having a pressure of 9-17 bar(e)) is required in order to reach a sufficiently high evaporation temperature in the final concentrator and to lower the viscosity of the liquor as well as to compensate the decrease in capacity caused by the considerable boiling point rise. Thus, the pressure and the temperature of the secondary vapor (its heat enthalpy) generated from the black liquor to be evaporated in the final concentrator will be so high that as regards heat economy, it is difficult to utilize this vapor as effectively as possible in the multi-effect evaporation plant described above, even though it would be a natural further use for the vapor. Feeding this kind of vapor into further effects operating at a lower dry solids content is not profitable, because as a heat source for these effects, vapor can be used having a lower pressure and temperature than the secondary vapor of the final concentrator.
It is an object of the invention to provide a method whereby the secondary vapor generated in the evaporation of the black liquor can be utilized more effectively as regards heat economy than before. In particular, it is a further object of the invention to provide a method of utilizing the secondary vapor that has been generated in the final evaporation of the black liquor to the dry solids content above 80%.
The present invention comprises a method whereby black liquor is concentrated in a multi-effect evaporation plant in order to obtain black liquor at a suitable dry-solids content for combustion and to generate vapor. The concentrated black liquor is taken to be combusted in a recovery boiler, into which combustion air is fed at several levels for the combustion process. It is a characteristic feature of the invention that the combustion air to be fed to at least one air level is preheated by using vapor generated in the concentration of the black liquor as a heat source.
The invention also relates to an arrangement for combusting black liquor from a pulp mill, the characteristic features of which being defined in the appended claims relating to the arrangement.
In the new process of the invention the vapor produced in the concentration of black liquor is utilized in an entirely new way as it is used to preheat the combustion air needed for the combustion of black liquor subsequent to the evaporation plant. Secondary vapor that is particularly suitable for this purpose is produced in the final concentration of the black liquor, whereby the dry solids content of the black liquor rises above 80% and whereby medium pressure steam having typically a pressure of 10-12 bar is used as heating medium.
When combusting concentrated black liquor, the purpose is to separate from each other the organic and inorganic constituents of the dry solids in the black liquor. The heat from the organic constituent of the dry solids is recovered and this heat is used to produce as much steam as possible. Process chemicals are recovered from the inorganic constituent of the dry solids in such a form that at later stages of the process they can be transformed to be reused in the cooking process. The concentrated black liquor is sprayed in a droplet form into the furnace of the recovery boiler. Also fed into the furnace of the recovery boiler is a stoichiometric amount of combustion air corresponding the amount of black liquor and also some surplus air to ensure complete combustion. Air is usually fed in at three levels: primary air at the bottom of the furnace, secondary air above the level of the primary air but below the liquor nozzles, and tertiary air above liquor nozzles in order to ensure complete combustion. In addition to these, there may also be other levels for feeding in air, e.g. above the tertiary air level, whereby the object is to decrease the amount of nitrogen oxides formed in the combustion.
Conventionally, at least the primary and secondary air is pre-heated before feeding into the furnace in order to maintain a sufficiently high combustion temperature in the furnace. Ambient air is used as the tertiary air and the air for the higher levels, and it is usually not heated. In the preheating the combustion air is heated typically to a temperature of approx. 150xc2x0 C. The heating process is usually divided into two stages, of which the first stage utilizes low pressure steam, e.g. at the pressure of approx. 3 bar, and the second stage uses steam at a higher pressure, e.g. at approx. 11 bar. When combusting black liquor having dry solids content above 80% in the recovery boiler, the temperature of the combustion air to be fed into the furnace of the boiler is lower than the corresponding temperature in the combustion of lower-solids black liquor. Typically, the temperature of the air is about 100xc2x0 C., which makes it possible to use steam having a lower pressure than the normal low pressure steam (approx. 3-4 bar(e)) of the mill in the air pre-heaters, and therefore, according to the invention, heat originating from the secondary vapor is utilized. Consequently, a steam pressure of approx. 0,5-1 bar(e) is sufficient in the air pre-heater.
The process according to the invention is very economical as regards heat consumption, because the heat used for the evaporation of the black liquor is recovered to be further used for heating the combustion air for the recovery boiler. In this way, the heat that is required for pre-heating the air may be used twice; first to evaporate the black liquor and then to pre-heat the air.
The process according to the invention is particularly suited for pulp mills, where black liquor has previously been concentrated to a dry solids content of 70-75%, but in order to enhance the combustion, the aim is to reach the level above 80%. In certain exemplary cases it has been shown that in additional evaporation where the black liquor has been concentrated further to a dry solids content of above 80%, the heat energy thus generated is approximately 70-90% of the energy needed to preheat the combustion air. The required additional heating of the combustion air is provided by heating separately with live steam which is usually needed in the system for start-ups and shutdowns and for regulating the process.
It is fairly uncomplicated to apply the invention in an existing plant as the number of connections to the evaporation plant and the boiler is small. Due to a lesser need for connections required for the realization of the arrangement according to the method of the invention, it is relatively easy to position the arrangement in connection with the recovery boiler. The heat consumption of the mill does not increase even though a new black liquor concentrator using medium pressure steam as heating medium is installed in the mill, because the vapor generated in the concentrator is utilized further in a heat-economical way, i.e. in the air pre-heaters. Additional operating costs resulting e.g. from a decrease in the production of electricity in the mill, which is caused by replacing the low pressure steam used in the pre-heating with the medium pressure steam used for evaporation, is merely a small portion in comparison with the yield from the additional steam production from the evaporation (i.e. secondary vapor from the concentrator).
The secondary vapor originating from the black liquor may include corroding compounds to such an extent that the material of which the air pre-heater is manufactured must have certain special properties, if the secondary vapor is led directly to the pre-heaters. According to a preferred embodiment of the invention, the secondary vapor is condensed in an evaporation unit (a reboiler) in which the condensing medium is demineralized water or the like. The steam thus generated is so pure that it may be fed into the same line with the live steam directed to the air pre-heaters. In the pre-heaters the air is condensed and the condensate is returned to the reboiler. When using the evaporation unit as an intermediate loop boiling demineralized water, no special arrangements are needed for the material of the air pre-heater. The pre-heating can be realized also without an intermediate loop (reboiler) if the secondary vapor used as a heat source does not contain to a damaging extent substances that can affect an air pre-heater manufactured of conventional material and having a conventional construction.